A first picture has already been released, complete with what looks like a press release. Kylie Minogue plays a character called Astrid, which I feel may be an in-joke to a certain season 5 story.
As reported here
12 July update: Gallifrey Guardian reports extra cast details:
"The BBC report also confirms casting details for the special, including two names reported earlier by Outpost Gallifrey, classic series veterans Geoffrey Palmer (who plays the Titanic's Captain) and Clive Swift. Additional actors in the episode are Gray O'Brien, Debbie Chazen, Clive Rowe, Russell Tovey, George Costigan and Jimmy Vee, previously seen in the programme as the Moxx of Balhoon in series one."